The present invention relates to a receptacle for containing a semi-solid substance such as a cream, a gel, or an emulsion. More particularly, the receptacle is designed to be used in association with a dispenser device of the type that prevents ingress of air, i.e. the volume of substance taken from the receptacle by the dispenser device is not replaced by a corresponding volume of air. Under such circumstances, two solutions can be envisaged: a first consists in packaging the semi-solid substance in a flexible bag which shrinks as the substance is dispensed by the dispenser device, which may be a pump for example. A second solution consists in fitting the receptacle with a follower piston which rises in sealed manner within the receptacle in steps each of a size corresponding to a volume of substance taken from the receptacle. It is thus the piston that produces the reduction in volume.
It is already known in the prior art to fit receptacles of round cross-section with such follower pistons. There is no particular technical difficulty in making a mold for a round follower piston, nor in using the mold, given that both the mold and the part to be molded are accurate bodies of revolution.
It is also known from the prior art to make receptacles of cross-section other than round. A receptacle of that type is particularly adapted for use with a dispenser device allowing ingress of air or for use with a flexible bag. This is because it is difficult to make a follower piston that is not round, since the sealing contact between the inside face of the receptacle and the sealing contact lip of the follower piston is no longer round. With a round piston, the sealing contact force is exerted radially and uniformly against the inside face of the receptacle. In contrast, with a non-round piston, it is not possible to obtain a good fit for the sealing contact or a uniform force over the inside periphery of the receptacle under the same conditions as apply with a perfectly round piston.
Like many other parts, follower pistons are made by molding, i.e. by injecting heated plastics material into a mold. As it cools, the initially expanded plastics material contracts, thereby leaving an empty gap. The gap is thus a direct physical consequence of heat loss due to contact between the hot plastics material and the cooler mold. This must be taken into account by the mold so as to obtain a plastics material part having the desired dimensions. Thus, the mold must be slightly larger than the desired final part so as to be able to compensate for the material shrinking. In practice, mold makers give a mold its final dimensions that take account of shrinkage by retouching the mold using appropriate tooling, e.g. a wire electro-erosion machine. This operation is performed empirically until a mold is obtained that has the appropriate dimensions, given shrinkage.
For a round follower piston, the retouching operation is relatively simple. Parts having axial symmetry, such as a round piston, are molded by injection at the center, i.e. hot semi-liquid plastics material penetrates radially into the mold and reaches the outermost portions corresponding to the sealing lips of the follower piston all at the same time. The shrinkage phenomenon therefore affects the molded part uniformly over its entire periphery. This simplifies retouching work since it must be performed uniformly at the periphery. The retouching machine can then be programmed in simple manner to work on a circle. This technique is well known in the prior art and is specially adapted for dimensioning molds for round follower pistons.
For making a follower piston that is other than round, such retouching work is still required. However, it is no longer possible to program the retouching machine with a working path that is as simple as a circle. Further, since the piston is not round, when the plastics material is injected at the center, it does not reach all of the extreme portions corresponding to the sealing lips simultaneously. The semi-liquid plastics material must travel along a path inside the mold which is not radially uniform. Since the piston is not round, there are necessarily extreme portions of the mold which are further than others from the center where injection takes place. Unfortunately, the shrinkage phenomenon is proportional to the thickness of the part. Portions further away from the injection center are therefore more severely affected by the shrinkage phenomenon than are other portions closer to the injection center. More retouching work is therefore required on such portions remote from the injection center. The complexity of the shape combined with the non-uniformity of the shrinkage phenomenon make retouching work practically impossible to perform. The retouching machine becomes unprogrammable.